Oil sands in the Athabasca region of northern Alberta constitute one of the largest hydrocarbon deposits in the world, containing about 173 billion barrels (bbls) of recoverable bitumen. Approximately 20% of this volume is surface mineable.
Mined oil sand is trucked to an ore preparation plant (OPP) where mined oil sand is crushed and further comminuted with the addition of hot water. If warranted, chemicals to enhance bitumen recovery are added to generate a slurry, which is then pipelined to an extraction plant.
The slurry is received and processed in the extraction plant, typically through a series of settling and flotation vessels where bitumen-rich froth is extracted from the bulk of the water and the solids in the slurry. The water, coarse solids and fine solids discharged from the extraction plant form large volumes of liquid tailings, typically comprising the fine solids having a diameter less than about 44 microns, and solid tailings, typically comprising the coarse solids having a diameter greater than about 44 microns. The bitumen-rich froth is further processed in a froth treatment plant to produce a final bitumen product and a smaller, froth treatment tailings stream comprising primarily fine solids and water.
The large volumes of coarse and fine solids and process water initially form a tailings slurry which is transported to tailings impoundment facilities. Oil sand mine operators are required to safely contain all solids derived from the tailings slurry and ultimately reclaim all disturbed land to a productive state. Operators are further required to retain any remaining fluid tailings throughout the life of a mine and, at the end-of-mine life, permanently store any residual fluid.
When the tailings slurry streams are impounded during normal operations, such as in one or more tailings ponds, coarser solids separate from the water in the slurry to form beaches above and below the water surface. Some of the fine solids in the slurry are captured in the sand beaches. The remainder of the fine solids typically report to the tailings ponds, suspended throughout the water column therein. In a steady-state operation, the concentration of the suspended solids achieves a vertical distribution ranging in the tailings ponds from about 0.5 wt % at the surface to about 30 wt % at the bottom of the pond. The fines in suspension at the bottom of the pond are called mature fine tailings (MFT). Historically, MFT have been found to consolidate at inconsequential rates relative to the lifetime of a mine, resulting in large inventory accumulations during mine operation.
The accumulation of MFT as a result of conventional tailings management operations has significant consequences:
(i) tailings storage volumes must be continually increased to accommodate the increasing MFT volumes;
(ii) significant quantities of water are retained in the MFT, resulting in an equivalent demand on fresh water intake or make-up to sustain the operation; and
(iii) provision for perpetual containment of the MFT in a safe, environmentally-acceptable manner, at the end of the mining operation.
As a consequence, the Energy Resources Conservation Board (ERCB) of Alberta, Canada, issued Directive 074, “Tailings Performance Criteria and Requirements for Oil Sands Mining Schemes” in 2009. The directive establishes stringent criteria for the reduction of fluid tailings and the formation of trafficable deposits, and stipulates a comprehensive protocol for reporting the performance of fine tailings deposits. In summary, Directive 074 requires that 50% of the fines in the processed oil sand ore feed be captured immediately in designated disposal areas (DDAs). Further, the fines deposited in the DDAs must achieve a minimum undrained shear strength of 5 kilopascals (kPa) in the materials deposited in the previous year and be ready for reclamation within 5 years after active deposition has ceased by ensuring that a trafficable surface layer of the deposit has a minimum undrained shear strength of 10 kPa.
Others have attempted to reduce the accumulation of MFT inventory. In the early 1990s, a collaboration by Suncor Energy Inc. (Calgary, Alberta, Canada), Syncrude Canada Ltd. (Fort McMurray, Alberta, Canada) and the University of Alberta (Edmonton, Alberta, Canada) established the basis for a new approach to control the accumulation of MFT. The approach involves creating a blend of coarse sand tailings, fines, water and a coagulating agent, typically gypsum, in which resulting coagulated fines are purported to have sufficient strength to prevent the sand from separating from the mixture. The mixture is called composite or consolidated tailings (CT) or non-segregating tailings (NST). The weight of the sand dispersed within the CT was thought to be sufficient to accelerate the dewatering of the fines in the CT. It has been purported that a competent surface amenable to reclamation can be attained using CT in less than a decade. Successful implementation of CT has not been straightforward. Significant development work has been done primarily by Suncor and Syncrude.
For operations having sizeable inventories of “legacy” MFT in tailings ponds, CT operations have not resulted in a reduction of MFT inventory, as new MFT continues to be produced at rates greater than that which can be used in CT production. Notwithstanding the apparent advance achieved with the discovery and implementation of CT, tailings storage volumes at the operating plants have continued to exceed approved containment volumes.
Other additives such as flocculants or carbon dioxide have been noted in the prior art to try to entrap the fines to release some of the water. Applicant believes that the use of these additives has been marginally successful to release water, but may not be sufficient to achieve Directive 074 targets without additional efforts such as thin lift drying, the use of additional mechanical equipment such as centrifuges, cyclones, thickeners, in-line treatment or additional chemical treatment.
The use of mechanical equipment treatment, as detailed above, with various discharge methods have been noted in prior art. While the use of mechanical equipment enhances the release of water from the fines prior to deposition, its use may not provide an economically attractive solution.
Canadian patent application 2,684,155 to EI DuPont de Nemours describes the use of a polysilicate microgel (PSM) to enhance (i) bitumen recovery, particularly from poor quality oil sands, and (ii) dewatering of fine tailings. The use of combinations of other chemical additives (polyacrylamides, preferably anionic, low molecular weight cationic polymers, multivalent metal compounds, silicates, NaOH, and sodium citrate) is also considered. The use of PSM results in more rapid filtration and higher solids concentration in the cake (up to 55% mineral). PSM addition is in the range 25-5000 g/t oil sand.
Canadian Patent Application 2,616,707 to Remedial Construction Services LLP describes the use of an ash composition for treating a wide range of sludges or solid materials having undesirably high moisture contents for forming a treated sludge capable of supporting the weight of commercial construction equipment. The suggested thickness of the final treated material is about 5 feet. The ash compositions suitable for embodiments described in Canadian Patent Application 2,616,707 comprise ash high in alumina, sulfate, calcium, ash formed during flue gas desulfurization, gypsum, or any other ash or mixtures of ash that include ingredients sufficient to form a calcium aluminum sulfate matrix or mixtures or combinations thereof. One of skill would understand that the ash described are mainly derived from the underflow of a combustion unit, being high in metal oxide sulfate and low in silica. The matrix takes up a varying amount of water, being 10 to 50 moles of water per mole of matrix mineral depending on pH. A hydrated calcium aluminum sulphate hydroxide (ettringite) matrix appears to be preferred. The described sludge reclamation requires relatively high dosage rates, being in the order of about 1 wt % to about 50 wt % of the entirety of the sludge being treated.
Applicant believes that many geopolymer additives used in construction and other industries are typically cement or are mixtures which comprise a large amount of cement with a smaller amount of fly ash. As such, these geopolymer additives are useful in treating sludges which comprise greater than about 40 wt % dry solid content and for forming high strength materials, such as having yield strengths in the megapascal range.
Typically, mature fine tailings from an oil sand operation comprise about 25-35 wt % dry solids content. Thickened fine tailings, generally the result of an in-line treatment of an underflow from a thickener, typically comprise from about 40 wt % to about 60 wt % dry solids content.
As taught in a paper titled FLY ASH—COAL COMBUSTION RESIDUE by Dr. Kamar Shah Ariffin for a course in Industrial Minerals (EBS 425/3) taught at the University of Malaysia, the entirety of which is incorporated herein by reference, fly ash is the finely divided mineral residue resulting from the combustion of ground or powdered coal in electric generating plants (ASTM C 618). The pertinent portions of the paper are largely reproduced herein as follows:                “Fly ash consists of inorganic matter present in the coal that has been fused during combustion of the coal. The particles solidify while suspended in the exhaust gases and are generally spherical in shape. The fly ash is collected in electrostatic precipitators and has a silt size of about 0.074 to about 0.005 mm.        Fly ash is a pozzolanic material and has been classified into two classes, F and C, depending upon the chemical composition of the fly ash. According to ASTM C 618, the chemical requirements to classify any fly ash are shown in the following Table A.        
TABLE AFly Ash ClassPropertiesClass FClass CSilicon dioxide (SiO2), plus aluminum oxide70.050.0(Al2O3) plus iron oxide (Fe2O3), Min. %Sulfur trioxide (SO3), Max %5.05.0Moisture Content, Max %3.03.0Loss on ignition, Max %6.0*6.0*The use of class F fly ash containing up to 12% loss of ignition may be approved by the user if acceptable performance results are available.                Class F fly ash is produced from burning anthracite and bituminous coals. The fly ash has siliceous or siliceous and aluminous material, which itself possesses little or no cementitious value but will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperature to form cementitious compounds (Chu et al, 1993)        Class C fly ash is produced normally from lignite and sub-bituminous coals and usually contains significant amounts of calcium hydroxide (CaO) or lime (Cockrell et al., 1970). Class C fly ash, in addition to having pozzolanic properties, also has some cementitious properties (ASTM C 618-99).        Chemical constituents of fly ash mainly depend on the chemical composition of the coal. However, fly ash that are produced from the same source and which have very similar chemical composition, can have significantly different ash mineralogies depending upon the coal combustion technology use. The different ash mineralogies cause the hydration properties and leaching characteristics to vary between generating facilities.        The amount of crystalline material versus glassy phase material depends upon the processes used in a particular power plant. The relative proportion of each, the size distribution, the chemical nature of the glassy phase, the type of crystalline material and the nature and percentage of unburned carbon are factors which can affect the hydration and leaching properties of the fly ash.        Formation of cementitious material by the reaction of free lime (CaO) with the pozzolans (AlO3, SIO2, Fe2O3) in the presence of water is known as hydration. The hydrated calcium silicate gel or calcium aluminate gel (cementitious material) can bind inert material together. For Class C fly ash, the calcium oxide reacts with the pozzolans in the fly ash itself. In the case of Class F fly ash, lime must be added for the hydration reaction to occur. The pozzolanic reactions are as follows:CA(OH2)→Ca+++2[OH]−CA+++2[OH]−+SiO2→CSH (gel)CA+++2[OH]−+Al2O3→CAH (gel)        Hydration of tricalcium aluminate in the ash provides one of the primary cementitious products in many ashes. The rapid rate at which hydration of the tricalcium aluminate occurs results in the rapid set of these materials and is the reason why delays in compaction result in lower strengths of the stabilized materials. The hydration chemistry of fly ash is very complex and stabilization must be based on physical properties of the treated soil and cannot be predicted by the chemical composition of the fly ash.        The total metals content of the fly ash depends on the composition of the coal. The potential for leaching the metals depends on the total metals content and the crystallinity of the fly ash. Metals in the glasseous phase are expected to leach at much lower rates than metals in the crystalline phase. For stabilized soil the leachability of metals not only depends on the property of the fly ash but also on the soil used. Some part of the metals leached from the fly ash will be absorbed on the clay minerals in the soil.”        
There is interest in the industry in finding a relatively low cost solution to the problems related to the accumulation and deposition of fluid fine tailings and which can be applied to fluid fine tailings regardless the manner in which they have been produced in an oil sand operation.